The present invention relates to a method and apparatus for providing high pressure fluid jet streams and, in particular, the invention relates to an orifice assembly for providing a highly cohesive fluid jet, e.g. a waterjet. Such fluid or water jets are now used for cutting of various materials, including hard materials such as stone and concrete, and softer materials such as, for example, plastics and leather.
In the past, a problem with devices producing high pressure fluid jets is that the cohesiveness of the jet, i.e., the convergence of the velocity vectors of the fluid making up the fluid jet, only extends for a relatively short distance. Being able to create a more cohesive or convergent fluid jet allows for finer fluid jet streams and, accordingly, more precise cutting, as well as the ability to allow the fluid jet nozzle to be disposed at a greater distance from the object being cut or to cut more deeply. This is particularly important in the robotics area, for example, where a fluid jet must closely follow the contour of the object being cut because of the small distance over which the fluid jet is cohesive. At greater distances from the object, the fluid jet becomes more turbulent, providing a wider kerf or width of cut, and, if too turbulent, thereby reducing the precision of the cut, or reducing the ability to cut the material at all. It has been observed that a reason for the lack of cohesiveness of a cutting jet is the presence of turbulence upstream of the orifice through which the cutting jet emerges. In addition to the above problems, the presence of turbulence may result in undesirable wetting of the material being cut.
Several devices have been proposed in the past for solving this problem. One is disclosed in U.S. Pat. No. 3,997,111, in which a lengthy liquid collimating device is disposed upstream of the nozzle orifice and wherein the flow collimating chamber is at least one hundred times greater than the cross-sectional area of the nozzle opening.
In another proposal, U.S. Pat. No. 4,852,800, a convergent section is disposed upstream of the orifice to reduce the turbulence upstream of the orifice and thereby provide a more convergent fluid jet downstream of the orifice.
Although the above devices help to provide a more cohesive fluid jet from the fluid jet orifice, they suffer from a number of disadvantages. The collimating chamber of the '111 patent is disadvantageous for its size and weight. The device of the '800 patent requires modifications to be made to the collimating chamber of the nozzle or fluid supply tube by the provision of a conical section upstream of the orifice.
In one commercially-available fluid jet producing device, the supply tube to the fluid jet producing orifice is approximately 3/16 inch. In another commercial design, the supply tube is approximately 1/4 inch. The larger, 1/4 inch supply tube provides less turbulence to the nozzle orifice than the 3/16 inch supply tube. The larger supply tube, therefore, provides a more cohesive fluid jet from the orifice than those devices provided with the smaller diameter supply tube.